1. Field of the Invention
The present invention is generally related to forming protective coatings on aluminum and aluminum alloys which will increase corrosion resistance by using chemicals that pose a relatively small environmental hazard and have a small toxic effect.
2. Description of the Prior Art
Metal surfaces are often protected from corrosion by the application of a barrier coating. A first category of barrier coatings are anodic oxides, and these types of coatings are usually formed by an electrochemical means known as "anodizing" during immersion in an inorganic acid like H.sub.2 SO.sub.4 or H.sub.3 PO.sub.4. Anodic oxides have a wide range of thicknesses and porosities. Porous coatings can be "sealed" in steam, boiling water or various salt solutions. A second category of barrier coatings are ceramic coatings, and these type of coatings are usually special cements applied to a metal to prevent corrosion. A common example of a ceramic coating is porcelain enamel. A third category of coatings are molecular barrier coatings, and these types of coatings are formed by the addition of organic molecules to solution. Effective inhibitors are transported to the metal-solution interface and have a reactive group attached to a hydrocarbon. The reactive group interacts with the metal surface while the hydrocarbon group is exposed to the environment. As the molecules form the molecular barrier coating, corrosion reactions are slowed. A fourth category of barrier coatings are organic coatings, and these types of coatings are generally intended to simply prevent interaction of an aggressive environment with the metal surface. Organic coatings are the most widely used barrier coatings for metals and paint is a typical example of an organic coating. A fifth category of barrier coatings are conversion coatings, and these types of coatings are made by a process which "converts" some of the base metal into the protective oxide coating. Chromate and phosphate conversion coatings are the two most common types of conversion coatings currently used.
Chromate and phosphate conversion coatings can be formed by chemical and electrochemical treatment of a metallic component during immersion in a solution containing hexavalent chromium (Cr.sup.+6), phosphorus as a phosphate anion, and usually other components. Literally hundreds of subtly different, proprietary chromate conversion coating formulas exist. For aluminum and aluminum alloys, the primary active ingredient in the bath is usually a chromate, dichromate (CrO.sub.4.sup.2- or Cr.sub.2 O.sub.7.sup.2-), or phosphate (PO.sub.4.sup.3-). The pH of the solutions is usually in the range of 1.3 to 2.5, but a few alkaline bath formulas are known. The process results in the formation of a protective, amorphous coating comprised of oxides of the substrate, complex chromium or phosphorus compounds, and other components of the processing solution. Only a small number of coatings and chromating processes have been characterized by surface analysis techniques. But in coating systems that have been studied, the following compounds have been reported: substrate oxides and hydroxides such as Al.sub.2 O.sub.3 and Al(OH).sub.3, chromium oxides and hydroxides such as Cr.sub.2 O.sub.3, CrOOH, Cr(OH).sub.3, and Cr.sub.2 O.sub.3 .multidot.xH.sub.2 O, and phosphates such as AlPO.sub.4. These coatings enhance corrosion resistance of bare and painted surfaces, improve adhesion of paint, or other organic finishes, or provide the surface with a decorative finish.
Chromate conversion coatings are applied by contacting the processed surfaces with a sequence of solutions. The basic processing sequence typically consists of the following six steps: cleaning the metal surface, rinsing, creating the conversion coating on the metal surface, rinsing, post treatment rinsing, and drying. The cleaning, rinsing, and drying steps are fairly standard procedures throughout the industry. The chief variant among the processes used is the composition of the chromate conversion solution. The compositions of these solutions depends on the metal to be treated and the specific requirements of the final product. The chief disadvantage of chromate conversion coating processes is that they involve the use of environmentally hazardous and toxic substances. It is expected that the use of substances like chromates will soon be regulated under stringent guidelines.
Because of the environmental problems with chromates, much work has been done to develop protective coatings which do not employ such compounds. For example, U.S. Pat. No. 4,004,951 to Dorsey discloses applying a hydrophobic coating on an aluminum surface by treatment with a long chain carboxylic acid and an equivalent alkali metal salt of the carboxylic acid, U.S. Pat. No. 4,054,466 to King et al. discloses a process for the treatment aluminum in which vegetable tannin is applied to the surface of the aluminum, and U.S. Pat. No. 4,063,969 to Howell et al. discloses treating aluminum with a combination of tannin and lithium hydroxide. In each of the above patents, the primary protective ingredient is the complex organic compound, the treatment solution is applied at slightly elevated temperatures (90.degree.-125.degree. F.), and the treatment solution is kept at a mid-level pH (4-8 in King and Howell, and 8-10 in Dorsey). Csanady et al., in Corrosion Science, 24, 3, 237-48 (1984) showed that alkali and alkali earth metals stimulated Al(OH).sub.3 growth on aluminum alloys. However, Csanady et al. report that the incorporation of Li.sup.+ or Mg.sup.+ into a growing oxide film degrades corrosion resistance.